Gas turbine engines, such as those used to power modern commercial aircraft, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. In aircraft engine applications, the compressor, combustor and turbine are disposed about a central engine axis with the compressor disposed axially upstream of the combustor and the turbine disposed axially downstream of the combustor.
Combustion of the hydrocarbon fuel in air in gas turbine engines inevitably produces emissions, such as oxides of nitrogen (NOx), carbon monoxide and hydrocarbons, which are delivered into the atmosphere in the exhaust gases from the gas turbine engine. It is generally accepted that oxides of nitrogen are produced at high flame temperatures. One approach to lower NOx emissions is to lower flame temperature by operating the combustor under fuel lean conditions. However, during operation of the combustor under fuel lean conditions, combustion instability and flame-out may occur if the fuel and air mixture becomes too fuel lean. Additionally, during operation of the combustor under fuel lean conditions, the lower flame temperatures could result in incomplete combustion and a consequent increase in carbon monoxide and hydrocarbons emissions.
Another approach to lower the emissions of oxides of nitrogen, carbon monoxide and hydrocarbons from a gas turbine engine is through staged combustion. One arrangement for implementing staged combustion in a gas turbine engine is to provide a plurality of fuel injection nozzles and associated air swirler assemblies, of which only a selected portion are operated at engine idle and under low power demands and all of which are operated at engine cruise and under high power demands.
In general, it is desirable to rapidly mix the fuel and the air in an attempt to provide uniform fuel lean conditions and eliminate as many local pockets as possible of combustion under near stoichiometric fuel/air conditions to avoid pockets of high flame temperature conducive to NOx formation, or of combustion under fuel rich conditions to avoid carbon monoxide and hydrocarbon resulting from incomplete combustion. Various designs of swirler assemblies have been developed for use in associated fuel injection nozzles in an attempt to provide rapid fuel and air mixing. For example, U.S. Pat. No. 5,966,937 discloses a fuel injector and a two-pass air swirler disposed about the fuel injector, the air swirler having an inner swirled air passage and an outer swirled air passage. The fuel is injected through the end of the fuel injector into the swirling airflow generated by the inner air swirler. U.S. Pat. No. 5,603,211 discloses a fuel injector and a three-pass air swirler disposed about the fuel injector, the air swirler having an inner swirled air passage, an intermediate swirled air passage and an outer swirled air passage. Again, the fuel is injected through the end of the fuel injector into the swirling airflow generated by the inner air swirler.
There is a desire for an efficient, low-emission, and stable combustor for use in gas turbine engines for powering supersonic cruise vehicles. It is contemplated that combustors in gas turbine engines for powering supersonic cruise vehicles will operate with pre-vaporized, that is gaseous, jet fuel. While the aforementioned air swirlers have performed well in mixing liquid jet fuel and air in conventional gas turbine engines on commercial subsonic aircraft, there is a desire for an air swirler assembly that provides rapid and efficient mixing of gaseous jet fuel with air.